1. Field of the Invention
The present invention is related to the field of electric wireline tools used to sample fluids contained within pore spaces of earth formations. More specifically, the present invention is related to methods of determining various properties of the earth formation by interpreting pressure readings made by electric wireline formation testing tools.
2. Description of the Related Art
Electric wireline formation testing tools are used to withdraw samples of fluids contained within pore spaces of earth formations and to make measurements of fluid pressures within the earth formation. Calculations made from these measurements can be used to assist in estimating the total fluid content within the earth formation.
Formation testing tools known in the an are typically lowered at one end of an armored electrical cable into a wellbore penetrating the earth formations. The formation testing tool typically comprises a tubular probe which is extended from a tool housing and then is impressed onto the wall of the wellbore. The probe typically is sealed on its outside diameter by an elastomeric packing element to exclude fluids from within the wellbore itself from entering the interior of the probe when fluids are withdrawn from the earth formation through the probe. The probe is selectively placed in hydraulic communication, by means of various valves, with sampling chambers included in the tool. Hydraulic lines which connect the probe to the various sample chambers can include connection to a highly accurate pressure sensor to measure the fluid pressure within the hydraulic lines. Other sensors in the tool can make measurements related to the volume of fluid which has entered some of the sample chambers during a test of a particular earth formation.
One of the properties of the earth formation which can be determined using measurements made by the wireline formation testing tool is permeability. Permeability is determined by, among other methods, calculating a rate at which a fluid having a known viscosity moves through the pore spaces within the formation when a predetermined differential pressure is applied to the formation. As previously stated, the formation testing tool typically includes a sensor to make measurements related to the volume of fluid entering the sample chamber, and further includes a pressure sensor which can be used to determine the fluid pressure in the hydraulic lines connecting the probe to the sample chamber. It is further possible to determine the viscosity of the fluid in the earth formation by laboratory analysis of a sample of the fluid which is recovered from the sample chamber.
In a method known in the art, the flow rate of the fluid from the formation into the sample chamber is typically determined by measuring the amount of time taken to fill the sample chamber, and calculating a flow rate by dividing the chamber volume by the measured time. The flow rate thus calculated can be used to calculate the permeability.
A drawback to the method known in the art for determining permeability from measurements made by the wireline formation test tool is that the test tools known in the art do not measure the sample chamber volume with sufficient accuracy and resolution in order to be able to determine that the flow rate calculated is representative of fluid flow only of the native fluid within the formation. To make measurements related to the volume of the sample chamber, the formation testing tools known in the art typically include means such as a direct-current "stepper" motor coupled to a screw drive, which moves a piston bounding one end of the sample chamber. It is typically not possible to control the volume change or the volume change rate caused by each one of the motor "steps". The testing tools known in the art include means for inferring the chamber volume by counting the number of motor steps, but by only counting steps, the testing tools known in the art can only indirectly determine the volume of the sample chamber. The volume of the chamber may therefore not be precisely known at any instant in time between the initiation of drawing a sample and the conclusion of drawing the sample. Subtle changes in the relationship of sample pressure to sample volume, which can be important in determining the permeability of the formation, can be obscured by the relatively low resolution of the test chamber volume measurement of the formation test tools known in the art. Subtle changes in the pressure/volume relationship of the sample can be affected by, among other things, the composition of the fluid actually withdrawn from the pore spaces of the formation.
Permeability which is calculated from measurements made of the pressure and the volume of the fluid being drawn into the chamber during the withdrawal of a sample can be affected by the composition of the fluid which is actually drawn into the chamber during draught of the sample. For example, when a wellbore is drilled through the earth formations, it is typically filled with a fluid having a specific gravity large enough so the fluid can exert hydrostatic pressure against the earth formation which can restrain native fluids within the formation from entering the wellbore. It is even more typical for the hydrostatic pressure of the fluid in the wellbore to at least slightly exceed the fluid pressure in the formation, so a part of the fluid within the wellbore, called "mud filtrate", typically is forced into the pore space in the formation by differential pressure. In addition, when the probe is first hydraulically connected to the sample chamber, it is still substantially filled with the fluid from within the wellbore, called "drilling mud". Both the drilling mud and the mud filtrate can have compressibilities and viscosities which are different from the fluid in the formation. Because the fluid which is actually drawn into the sample chamber will probably contain at least some drilling mud and mud filtrate, a formation permeability determination based only on the time taken for the sampled fluid to fill the volume of the sample chamber therefore can be erroneous because the flow rate thus determined can be in error.
The drawback to the formation test tools known in the art as described herein can be better understood by referring to FIGS. 1A and 1B. FIG. 1A is a graphic representation of fluid pressure with respect to time shown as curve 210, and is a graphic representation of volume with respect to time shown as curve 212. When a sample is first drawn, as shown beginning on curve 210 at a point indicated by reference numeral 214, the volume of the test chamber is increased. Some of the increase in chamber volume is dissipated by reducing the pressure of fluid in the hydraulic lines so that the hydraulic line pressure balances the pressure of the fluid in the formation, as indicated at the point shown at reference numeral 216. As the sample chamber volume increases further, the chamber pressure drops below the formation pressure and flow from the formation into the chamber begins. However, some of the fluid in the formation near the probe can be the "mud filtrate" previously described herein. The mud filtrate can have different compressibility and viscosity than does the native fluid in the formation. Consequently, the relationship of chamber pressure to chamber volume can be different when the fluid being drawn into the chamber consists of mud filtrate, as can be observed on curve 210 between points indicated with reference numerals 216 and 218. After the pressure drop in the formation caused by the increasing chamber volume is finally communicated to the fluid in the formation, as indicated on curve 210 at the point having numeral 218, the fluid movement into the chamber with respect to increasing chamber volume is affected principally by the properties of the fluid in the formation, as indicated between points 218 and 220 on curve 210. At point 220, the chamber has been expanded to a predetermined maximum volume, and the pressure in the chamber begins to increase as formation fluid continues to flow into the chamber. The flow will continue until the chamber pressure equals the formation pressure.
FIG. 1B shows the relationship of chamber pressure with respect to chamber volume. Curve 222 is a graphic representation of the relationship of pressure to volume for the sample test shown as related to time in FIG. 1A. For example, the previously referred to expansion of drilling mud in the probe and hydraulic line is shown between points 224 and 226; the expansion of the mud filtrate in the formation pore spaces is shown between points 226 and 228; and the portion of the chamber volume being filled by native fluid flow in the formation is shown between points 228 and 230.
The formation testing tools known typically do not have means for determining the volume of the chamber at intermediate points, such as 226 and 228 in curve 222 in FIG. 1B, to a sufficient degree of precision to determine the amount of flow corresponding only to the formation fluid.
Accordingly it is an object of the present invention to provide a formation test tool having a means for resolving the volume of the test chamber to a sufficient degree of accuracy to enable determining whether the fluid flowing into the formation test tool is caused by fluid movement from the pore space of the formation.
It is a further object of the present invention to provide a method of calculating permeability of the hydraulic zone by measuring the flow rate of fluid into a sample chamber of a wireline formation test tool after determining that the fluid flowing into the test chamber is caused to flow by movement of formation fluid in the pore space of the formation.